Method and Apparatus for Liquefying a Natural Gas Stream

ABSTRACT

The present invention relates to a method of liquefying a natural gas stream, wherein the natural gas stream ( 10 ) is provided at a pressure of 30-80 bar, expanded to a pressure &lt;35 bar, supplied to a gas/liquid separator ( 31 ) and therein into a vaporous stream ( 40 ) and a liquid stream ( 30 ). The pressure of the vaporous stream is increased to a pressure of at least 70 bar and the pressurized vaporous stream ( 90 ) is liquefied to obtain a liquefied natural gas stream.

The present invention relates to a method of liquefying a natural gas stream.

Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at high pressures.

Examples of known methods of liquefying gas are disclosed in e.g. U.S. Pat. No. 6,272,882 and DE 102 26 597 A1.

According to FIG. 1 of DE 102 26 597 A1 a natural gas stream having a pressure of 70-100 bar is expanded (Expander X) to a pressure range of 40-70 bar, cooled (Heat Exchanger E1) and fed to a Heavy Hydrocarbon (HHC) column (T1). A C₂-rich fraction taken from the overhead of the HHC column is further cooled (E2) and fed to a further column (D). The overhead stream of this further column (D) is pressurized (V) to a pressure in the range of 50-100 bar and subsequently liquefied.

A problem of the method according to DE 102 26 597 is that it is unnecessarily complicated. A further problem of the above method is that the recovery of compounds heavier than methane (in particular propane and butane) is insufficient.

It is an object of the present invention to minimize the above problems.

It is a further object of the present invention to increase the recovery of compounds heavier than methane, in particular propane.

It is an even further object of the present invention to provide an alternative method for liquefying a natural gas stream.

One or more of the above or other objects are achieved according to the present invention by providing a method of liquefying a natural gas stream, the method comprising the steps of:

(a) providing a feed stream containing natural gas at a pressure of 30-80 bar;

(b) expanding the feed stream of step (a), thereby obtaining an expanded feed stream having a pressure <(lower than) 35 bar;

(c) supplying the expanded feed stream to a gas/liquid separator;

(d) separating the expanded feed stream in the gas/liquid separator into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream;

(e) increasing the pressure of the vaporous stream obtained in step (d) to a pressure of at least 70, preferably at least 84 bar;

(f) liquefying the pressurized vaporous stream obtained in step (e) thereby obtaining a liquefied natural gas stream;

wherein the pressure of the feed stream provided in step (a) is not increased until the increase of pressure in step (e).

It has surprisingly been found that using the method according to the present invention, a significantly increased recovery of compounds heavier than methane can be obtained. An important advantage of the present invention is that this can be achieved in a surprisingly simple manner.

A further advantage of the present invention is that an increased production of liquefied natural gas can be obtained using a given refrigeration power. Thus, for a given refrigeration power (e.g. using a given line-up comprising one or more cryogenic heat exchangers, compressors, etc.) the method according to the present invention provides more LNG than a known process. It has been found that according to the present invention increases in LNG product as high as 20% may be obtained, while keeping the refrigeration power constant.

The natural gas stream may be any suitable gas stream to be liquefied, but is usually obtained from natural gas or petroleum reservoirs. As an alternative the natural gas may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.

Usually the natural gas stream is comprised substantially of methane. Preferably the feed stream comprises at least 60 mol % methane, more preferably at least 80 mol %, most preferably the feed stream comprises at least 90 mol % methane.

Depending on the source, the natural gas may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes as well as some aromatic hydrocarbons. The natural gas stream may also contain non-hydrocarbons such as H₂O, N₂, CO₂, H₂S and other sulphur compounds, and the like.

If desired, the feed stream containing the natural gas may be pre-treated before it is expanded and fed to the gas/liquid separator. This pre-treatment may comprise removal of undesired components such as CO₂ and H₂S, or other steps such as pre-cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, they are not further discussed here.

The gas/liquid separator may be any suitable means for obtaining a vaporous stream and a liquid stream, such as a scrubber, distillation column, etc. If desired, two or more gas/liquid separators may be present.

The person skilled in the art will readily understand that the increase in pressure of the vaporous stream may be performed in various ways, provided that a pressure of at least 70, preferably at least 84 bar is obtained.

Also, the person skilled in the art will understand that the liquefaction of the pressurized vaporous stream may be performed in various ways, e.g. using one or more cryogenic heat exchangers.

Further the person skilled in the art will readily understand that after liquefaction, the liquefied natural gas may be further processed, if desired. As an example, the obtained LNG may be depressurized by means of a Joule-Thomson valve or by means of a cryogenic turbo-expander. Also, further intermediate processing steps between the gas/liquid separation and the liquefaction may be performed.

Preferably in step (e) the pressure is increased to at least 86 bar, preferably at least 90 bar. Herewith the amount of LNG product obtained may be increased. As a result of the relatively high pressure used, the vaporous stream may be supercritical, depending on the prevailing pressure and the composition of the respective vaporous stream. Preferably the vaporous stream is supercritical, as this avoids phase changes in the liquefaction process.

Further it is preferred that the vapour stream obtained in step (b) has a C₅+ content of below 0.5 mol %, preferably below 0.1 mol %. This minimizes operating problems in the downstream liquefaction unit. With “C₅+ content” is meant the content of hydrocarbon components having five or more carbon atoms.

According to a preferred embodiment, the pressure in step (e) is increased by compressing the vaporous stream in a compressor, thereby obtaining a compressed stream. To this end one or more compressors may be used.

Further it is preferred that the vaporous stream obtained in step (e) is cooled, e.g. in an ambient heat exchanger. Further it is preferred that the compressed stream is heat exchanged against the vaporous stream obtained in step (d).

According to a particularly preferred embodiment of the method according to the present invention, an expander for expanding the feed stream in step (b) is functionally coupled to a compressor for compressing the vaporous stream. As a result, the power generated by the expander is used at least partially for driving the compressor to which it is functionally coupled. Hereby, the expander and compressor form a so-called “compressor-expander scheme”, as a result of which the energy consumption of the whole process is minimized. As the person skilled in the art will readily understand what is meant with a “compressor-expander scheme”, this is not further discussed here.

In a further aspect the present invention relates to LNG product obtained by the method according to the present invention, in particular liquefied methane.

In an even further aspect the present invention relates to an apparatus suitable for performing the method according to the present invention, the apparatus at least comprising:

means for providing a feed stream containing natural gas at a pressure of 30-80 bar;

an expander for expanding the feed stream thereby obtaining an expanded feed stream having a pressure of <35 bar;

a gas/liquid separator for separating the expanded feed stream into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream;

a pressurizing unit for increasing the pressure of the vaporous stream obtained in the gas/liquid separator to a pressure of at least 70, preferably at least 84 bar; and

a liquefaction unit for liquefying the vaporous stream having a pressure of at least 70, preferably at least 84 bar, the liquefaction unit comprising at least one cryogenic heat exchanger.

Preferably, the pressurizing unit comprises a compressor.

Further it is preferred that the apparatus further comprises a heat exchanger for heat exchanging an effluent from the compressor against the vaporous stream obtained in the gas/liquid separator.

Also, apparatus preferably further comprises an expander for expanding the feed stream.

According to a particularly preferred embodiment, the compressor and expander are functionally coupled, thereby forming a so-called “compressor-expander scheme”.

Hereinafter the invention will be further illustrated by the following non-limiting drawing. Herein shows:

FIG. 1 schematically a process scheme in accordance with an embodiment of the present invention; and

FIG. 2 schematically a process scheme in accordance with another embodiment of the present invention.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components.

FIG. 1 schematically shows a base load liquefied natural gas (LNG) export process and an apparatus (generally indicated with reference number 1) for performing the same. A feed stream 10 containing natural gas is supplied to a gas/liquid separator 31 at a certain inlet pressure and inlet temperature, after being expanded in expander 12. Typically, the pressure of stream 10 will be between 30 and 80 bar (preferably >60 bar and <70 bar), and the temperature will be close to ambient temperature, usually between 5 and 50° C.

If desired the feed stream 10 may have been pre-treated before it is fed to the expander 12. As an example, the feed stream 10 may be pre-cooled against a refrigerant in a heat exchanger (not shown), or in a train of heat exchangers, for instance comprising two or more heat exchangers operating at different refrigerant pressure levels.

The expansion in expander 12 is chosen to form a partially condensed expanded feed stream 25. Further, the expansion in expander 12 is chosen to optimise a subsequent separation step in separator 31.

Expanded stream 25 is fed to the gas/liquid separator 31. There the feed stream in line 25 is separated into a vaporous overhead stream 40 and a liquid bottom stream 30. The overhead stream 40 is enriched in methane (and usually also ethane) relative to the expanded feed stream 25.

The bottom stream 30 is generally liquid and usually contains some components that are freezable when they would be brought to a temperature at which methane is liquefied. Separator 31 can be a separator vessel or a distillation column such as a scrub column, depending on the separation required to remove freezable components from the feed stream. Typically the freezable components are CO₂, H₂S and hydrocarbon components having the molecular weight of pentane or higher. These freezable components may also at least partially have been removed from the feed stream before entering the separator 31.

The bottom stream 30 may also contain hydrocarbons that can be separately processed to form liquefied petroleum gas (LPG) products.

Usually, the bottom stream 30 is subjected to one or more fractionation steps to collect various natural gas liquid products. The overhead stream 40 is compressed via compressor 52 thereby obtaining a compressed stream.

The compressed stream is discharged at a pressure above 70, preferably above 84 bar into line 65. The pressure-increase in this compression step is chosen between 30 bar and 150 bar, depending on the choices of respectively the separation pressure and the liquefaction pressure.

Part of the heat added during this compression step is removed from stream 65 against the ambient, for instance using an air cooler 61 or a water cooler. The resulting ambient-cooled stream 75 is then further cooled in one or more external cooling stages. This may include a pre-cooling stage, here depicted as heat exchanger 81. A train of subsequent heat exchangers may be employed instead.

A pre-cooled stream 90 is then further cooled into liquefaction in a liquefaction unit 5 at least comprising a main cryogenic heat exchanger 91. Any suitable type of heat exchanger may be employed. Here depicted is a cryogenic heat exchanger 91 operated by a mixed refrigerant, of which light and heavy fractions are first autocooled in tubes running parallel to the pre-cooled stream (not shown) and then expanded to the shell side via inlet means 95 and 96 respectively. The spent heavy and light fractions are drawn from the shell side of the main cryogenic heat exchanger 91 via outlet 97. The spent refrigerant in line 97 can be recompressed and cooled to form a liquid, or, in case of a mixed refrigerant, a mixed vaporous light fraction and liquid heavy fraction.

Referring again to stream 65, the liquefaction pressure is chosen to exceed a pressure of at least 84 bar, more preferably above 86 bar. As a result, the vapour in stream 65 may be in a supercritical condition.

As a next step, the liquefied stream leaving the main cryogenic heat exchanger 91 via line 100 is further cooled in a flash step wherein the pressure is let down via a valve or liquid expander 101. Suitably the pressure after expanding is about atmospheric. Expansion heat is extracted from the liquefied stream, so that the temperature is further lowered to a temperature under which the liquefied product remains liquid at atmospheric pressure. Flash gas 130, typically containing nitrogen and some methane, is separated from the stream 110 in flash tank 111. A part of the flash gas 130 can be employed as fuel gas for providing energy to the liquefaction process. The liquid part of stream 110 is discharged from the bottom of flash tank 111 in line 120. This can be stored and transported as LNG.

Preferably, the compressor train 52 uses expansion energy from at least expander 12. To this end at least one compressor of the compressor train 52 is functionally coupled to the expander 12 thereby forming a so-called “compressor-expander scheme”. Additional compression power may however be provided to achieve a pressure above 84 bar. Preferably, the additional compressor motor power consumed by the compressor 52 is chosen close to or identical to the power required by the refrigerant compressors (not shown) so that identical drivers can be employed for both purposes thereby providing cost and maintenance benefits.

Unlike the embodiment of FIG. 2, no heat integration (as in heat exchanger 41 in FIG. 2) is applied in the embodiment of FIG. 1 on the cold vested in overhead stream 40, so that after cooling the compressed overhead stream in line 65 against about ambient (in cooler 61) it is directly submitted via line 75 to external cooling steps in heat exchanger 81.

Table I gives an overview of the pressures and temperatures of a stream at various parts in an example process of FIG. 1. Also the mol % of methane is indicated. The feed stream in line 10 of FIG. 1 comprised approximately the following composition: 80% methane, 8% ethane, 5% propane, 4% butanes, 1% C₅+ and 2% N₂. Freezable components such as H₂S, CO₂ and H₂O were previously removed.

TABLE I Temperature Mol % Line Pressure (bar) (° C.) methane 10 67 32 80 25 32.8 −30 80 40 30.4 50.6 90 65 93 160.8 90 75 92.6 51 90 90 89 −41.5 90 100 81.5 −151.3 90 110 5.4 −157.8 90

FIG. 2 schematically depicts an alternative embodiment of the process according to the invention.

In this embodiment, the overhead stream 40 is led through an effluent stream heat exchanger 41, where it is indirectly heated against a stream of about ambient temperature (stream 70). Stream 50, which is discharged from the effluent stream heat exchanger 41 is then compressed via compressor 52 or a train of two or more compressors. The compressed stream is discharged at a pressure above 84 bar into line 60, cooled in e.g. an air cooler 61, thereby obtaining stream 70. The resulting ambient-cooled stream 70 is then led to the effluent stream heat exchanger 41 where it is cooled in indirect heat exchange with the cold overhead stream 40 thereby obtaining stream 80 which is further cooled in heat exchanger 81.

Table II gives an indication of increase in propane and butane recovery using the process as described in FIG. 1 according to the present invention. As a comparison the same line-up as FIG. 1 was used, but—in contrast to the present invention—an expansion to about 45 bar took place in expander 12. As shown in Table II the present invention results in an increased propane and butane recovery in stream 30 (16% and 36% versus 9% and 20% respectively).

TABLE II Line 30 30 (expansion in (expansion in line 25 to line 25 to 10 32.8 bar) about 45 bar) Flow 1 0.041 0.025 [kmol/s] Propane 0.05 0.194 0.173 [mol. fraction] i-Butane 0.02 0.155 0.137 [mol. fraction] Butane 0.02 0.196 0.176 [mol. fraction] Recovery Propane 16%  9% Butane 36% 20%

Table III gives an indication of increase in LNG product using the process as described in FIG. 1 according to the present invention. As a comparison, the same refrigeration power and line-up of FIG. 1 was used, but—in contrast to the present invention—no compression took place in compressor train 52; as a result, the pressure for the comparison in line 65 was the same as in line 40, i.e. about 30.4 bar. As can be seen from Table III the increase in LNG product was about 19%.

TABLE III Comparison - no FIG. 1 according compression in to the present compressor train Property (unit) invention 52 of FIG. 1 Power provided to 80 80 refrigeration cycle of exchanger(s) 81 (MW) Power provided to 80 80 refrigeration cycle of exchanger(s) 91 (MW) Combined size of 61,500 61,500 cryogenic heat exchanger 81 and 91 (kW/K) LNG production 13,169 11,080 (tpd) 

1. A method of liquefying a natural gas stream, the method comprising the steps of: (a) providing a feed stream containing natural gas at a pressure of 30-80 bar; (b) expanding the feed stream of step (a), thereby obtaining an expanded feed stream having a pressure <35 bar; (c) supplying the expanded feed stream to a gas/liquid separator; (d) separating the expanded feed stream in the gas/liquid separator into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream; (e) increasing the pressure of the vaporous stream obtained in step (d) to a pressure of at least 70 bar; (f) liquefying the pressurized vaporous stream obtained in step (e) thereby obtaining a liquefied natural gas stream; wherein the pressure of the feed stream provided in step (a) is not increased until the increase of pressure in step (e).
 2. The method according to claim 1, wherein in step (e) the pressure is increased to at least 86 bar.
 3. The method according to claim 1, wherein the vapour stream obtained in step (d) has a C₅+ content of below 0.5 mol %.
 4. The method according to claim 1, wherein the pressure in step (e) is increased by compressing the vaporous stream, thereby obtaining a compressed stream.
 5. The method according to claim 1, wherein the vaporous stream obtained in step (e) is cooled.
 6. The method according to claim 4, wherein the compressed stream, before it is liquefied in step (f), is heat exchanged against the vaporous stream obtained in step (d).
 7. The method according to claim 1, wherein an expander for expanding the feed stream in step (b) is functionally coupled to a compressor for compressing the vaporous stream.
 8. An apparatus for liquefying a natural gas stream, the apparatus at least comprising: means for providing a feed stream containing natural gas at a pressure of 30-80 bar; an expander for expanding the feed stream thereby obtaining an expanded feed stream having a pressure of <35 bar; a gas/liquid separator for separating the expanded feed stream into a vaporous stream and a liquid stream, the vaporous stream being enriched in methane relative to the feed stream, and the liquid stream being reduced in methane relative to the feed stream; a pressurizing unit for increasing the pressure of the vaporous stream obtained in the gas/liquid separator to a pressure of at least 70 bar, and a liquefaction unit for liquefying the vaporous stream having a pressure of at least 70 bar, the liquefaction unit comprising at least one cryogenic heat exchanger.
 9. The apparatus according to claim 8, wherein the pressurizing unit comprises a compressor.
 10. The apparatus according to claim 9, wherein the apparatus further comprises a heat exchanger for heat exchanging an effluent from the compressor against the vaporous stream obtained in the gas/liquid separator.
 11. The apparatus according to claim 9, wherein the compressor and expander are functionally coupled.
 12. The apparatus according to claim 8, wherein no further pressurizing unit is present between the means (for providing the feed stream at a pressure of 30-80 bar and the pressurizing unit
 52. 13. The method according to claim 1, wherein in step (e) the pressure is increased to at least 84 bar.
 14. The method according to claim 1, wherein in step (e) the pressure is increased to at least 90 bar.
 15. The method according to claim 2, wherein the vapour stream obtained in step (d) has a C₅+ content of below 0.5 mol %.
 16. The method according to claim 13, wherein the vapour stream obtained in step (d) has a C₅+ content of below 0.5 mol %.
 17. The method according to claim 14, wherein the vapour stream obtained in step (d) has a C₅+ content of below 0.5 mol %.
 18. The method according to claim 1, wherein the vapour stream obtained in step (d) has a C₅+ content of below 0.1 mol %.
 19. The method according to claim 2, wherein the vapour stream obtained in step (d) has a C₅+ content of below 0.1 mol %.
 20. The method according to claim 13, wherein the vapour stream obtained in step (d) has a C₅+ content of below 0.1 mol %. 